Alkylation of aromatic compounds with a C.sub.2 to C.sub.4 olefin and transalkylation of polyalkylaromatic compounds are two common reactions for producing monoalkyl aromatic compounds. Examples of these two reactions that are practiced industrially to produce ethylbenzene are the alkylation of benzene with ethylene and the transalkylation of benzene and a diethylbenzene. A simplified summary of the alkylation reaction and its common product and by-products is given below: ##STR1##
Although the formation of the diethylbenzene and triethylbenzene isomers might, at first glance, be viewed as by-products that represent a reduction in the efficient utilization of ethylene, in fact each can be readily transalkylated by benzene to produce ethylbenzene, as shown below: ##STR2## Combining alkylation and transalkylation can thus maximize ethylbenzene production. Such a combination can be carried out in a process having two reaction zones, one for alkylation and the other for transalkylation, or in a process having a single reaction zone in which alkylation and transalkylation both occur. In many cases, a single reaction zone is preferred over two reaction zones because of the savings in capital investment.
One disadvantage of alkylation-transalkylation processes, regardless of whether the alkylation and transalkylation reactions occur in the same or separate reaction zones, is that by-product 1,1-diphenylethane (1,1-DPE) can not be converted to ethylbenzene by alkylation or transalkylation, and thus 1,1-DPE represents a reduction in ethylene utilization efficiency and a loss of ethylene. In fact, the by-production of 1,1-DPE, as well as of the heavier polyethylated benzenes other than diethylbenzene and triethylbenzene which are collectively referred to herein as heavies, represents virtually all of the reduction in the ethylene utilization. The current minimum requirement for combination processes is that 1,1-DPE be not more than 1.0 wt-% relative to ethylbenzene. The formation of 1,1-DPE is assuming added importance and significance in view of the expectation in some areas of near-term minimum standards for the content of 1,1-DPE of not more than 0.5 wt-%.
In reaction zones where alkylation and transalkylation occur, it is known that the formation of 1,1-DPE depends in part on two key operating variables. The first operating variable is the molar ratio of phenyl groups per ethyl group, which is often referred to herein as the phenyl/ethyl ratio. The numerator of this ratio is the number of moles of phenyl groups passing through the reaction zone during a specified period of time. The number of moles of phenyl groups is the sum of all phenyl groups, regardless of the compound in which the phenyl group happens to be. For example, one mole of benzene, one mole of ethylbenzene, and one mole of diethylbenzene each contribute one mole of phenyl group to the sum of phenyl groups. The denominator of this ratio is the number of moles of ethyl groups passing through the reaction zone during the same specified period of time. The number of moles of ethyl groups is the sum of all ethyl and ethenyl groups, regardless of the compound in which the ethyl or ethenyl group happens to be. For example, one mole of ethylene and one mole of ethylbenzene each contribute one mole of ethyl group to the sum of ethyl groups, whereas one mole of diethylbenzene contributes two moles of ethyl groups.
The second operating variable that affects the 1,1-DPE formation is the concentration of ethylene in the alkylation zone. A practical, mathematical approximation is that the concentration of ethylene depends on the molar ratio of phenyl groups per ethyl group according to the formula: EQU [ethylene].apprxeq.[phenyl/ethyl ratio].sup.-1.
Thus, increasing the phenyl/ethyl ratio decreases the concentration of ethylene.
It is known that a low concentration of ethylene or a high molar ratio of phenyl groups per ethyl group minimizes formation of 1,1-DPE. The amount of 1,1-DPE formed depends on the phenyl/ethyl ratio according to the formula: EQU [1,1-DPE].apprxeq.[phenyl/ethyl ratio].sup.-2.
Thus, increasing the phenyl/ethyl ratio decreases the amount of 1,1-DPE formed. Although the decrease in 1,1-DPE formation that is conferred by a small increase in phenyl/ethyl ratio may be small, it also is very significant, resulting in a high phenyl/ethyl ratio being the condition of choice for minimizing 1,1-DPE formation. However, a high phenyl/ethyl ratio increases capital and operating costs that are usually associated with the recovery of excess benzene. These costs give impetus to a search for an ethylbenzene process that minimizes 1,1-DPE formation at a low phenyl/ethyl ratio.
In the prior art, the search for a commercially-viable alkylation process that not only produces a small amount of 1,1-DPE but also operates at a low phenyl/ethyl ratio in the alkylation zone has not been fruitful. All of the prior art processes follow the same, well-known approach of dividing the reaction zone into more and more catalyst beds and injecting smaller and smaller portions of the total ethylene into each bed. Where the allowed concentration of 1,1-DPE is relatively high, this approach undoubtedly confers some benefits. For example, if benzene is alkylated with ethylene in a single-bed alkylation zone that operates at a phenyl/ethyl molar ratio of 5, then the highest concentration of ethylene, which occurs at the point of ethylene injection, is 16.7 mol-%. Downstream of the ethylene injection point, the ethylene concentration decreases to very low concentrations as ethylene is consumed and ethylbenzene is formed, while the phenyl/ethyl ratio remains essentially the same. However, if the single bed is divided into four beds in series and if one-fourth of the required ethylene is injected into each bed, then the phenyl/ethyl ratio is 20 in the first bed, 10 in the second bed, 6.7 in the third bed, and 5 in the fourth bed. Accordingly, the highest concentration of ethylene is 4.8 mol-% in the first bed, 4.5 mol-% in the second bed, 4.3 mol-% in the third bed, and 4.2 mol-% in the fourth bed. Thus, dividing the bed and splitting the ethylene injection increases the phenyl/ethyl ratio and decreases the highest ethylene concentration.
But, in order to operate at the low phenyl/ethyl ratios and to also attain the low concentrations of 1,1-DPE that are expected to become the minimum standard in the near future, this prior art approach is not viable. For example, if benzene is alkylated with ethylene in a four-bed alkylation zone that operates at an overall phenyl/ethyl molar ratio of 2 rather than 5 as in the previous example, then the phenyl/ethyl ratio ranges from 8 in On the first bed to 2 in the fourth bed, and the highest ethylene concentration ranges from 11.1 mol-% in the first bed to 8.3 mol-% in the fourth bed. Compared to the previous example, the ethylene concentration in each bed approximately doubled, which would result in an unacceptable amount of 1,1-DPE formation. In order to reduce the ethylene concentrations to those in the previous example, the number of beds would have to be increased from 4 to 10, simply as a consequence of the fact that the overall phenyl/ethyl ratio had decreased from 5 to 2.
Thus, in response to industry's demands for lower phenyl/ethyl ratios and the market's demand for lower ethylene concentrations, the prior art process inexorably divides the reaction zone into a large number of very small catalyst beds. Because of a variety of technical, economic, and practical considerations, this inefficient solution by the prior art processes is unacceptable in the hydrocarbon processing industry.